1. Field of the Invention
The present invention relates to a strobe light control circuit for a camera, and more particularly to an insulated gate bipolar transistor (IGBT) for a strobe light.
2. Description of the Background Art
Insulated gate bipolar transistors (hereinafter referred to as “IGBT”) in 8-pin IC packages, for example, are widely employed as semiconductor elements for controlling strobe lights of cameras.
FIG. 4 is a top view showing the configuration of a conventional IGBT device for controlling a strobe light. A common IGBT device in TSSOP-8 package is illustrated here as an example. A semiconductor chip (IGBT chip) 10 has a gate electrode 11 and an emitter electrode 12 formed on its upper surface and a collector electrode (not shown) formed on its lower surface. The IGBT chip 10 is mounted on a lead frame 13 and bonded thereto by solder or the like such that the collector electrode formed on its lower surface is electrically connected to the lead frame 13. That is, the lead frame 13 functions as an external connection terminal (collector terminal) for the collector electrode of the IGBT chip 10.
A gate terminal 14 is an external connection terminal for the gate electrode 11 and is connected to the gate electrode 11 via a wire 14a made of metal. An emitter terminal 15 is an external connection terminal for the emitter electrode 12 and is connected to the emitter electrode 12 via wires 15a made of metal.
The above-described components are enclosed in a package 16 made of resin indicated by broken lines in FIG. 4 to constitute one IGBT device. In FIG. 4, the numbers 1 through 8 indicate the pin numbers of the IGBT device.
FIG. 5 is a circuit diagram showing an exemplary strobe light control circuit to which the conventional IGBT device shown in FIG. 4 is adopted. In the drawing, nodes that correspond to the components shown in FIG. 4 are designated by the same reference characters for ease of description. As shown in FIG. 5, a light emitter 20, an electrolytic capacitor 21, a switch 22 and a voltage source 23 are connected between the lead frame 13 as the collector terminal of the IGBT device and the emitter terminal 15. A drive circuit 24 for generating a voltage signal for driving the IGBT device is connected between the gate terminal 14 and emitter terminal 15. Resistances R14a and R15a are wiring resistances provided for the wires 14a and 15a, respectively.
Hereinafter, the operation of the conventional strobe light control circuit will be described. First, the switch 22 is turned on to cause the voltage source 23 to apply a predetermined value of voltage across the electrolytic capacitor 21. Therefore, a necessary amount of charges for the light emitter 20 to emit light is stored in the electrolytic capacitor 21. At this time, the IGBT chip 10 is in the OFF state.
Once a sufficient amount of charges is stored in the electrolytic capacitor 21, the drive circuit 24 applies a voltage pulse (voltage signal) having a predetermined amplitude on the basis of the voltage at the emitter terminal 15 to the gate terminal 14. This brings the IGBT chip 10 into the ON state. Then, the charges stored in the electrolytic capacitor 21 flows into the light emitter 20 (i.e., a large current flows), allowing the light emitter 20 to emit light brightly for illuminating a subject.
As described, when the IGBT chip 10 is turned on and a large amount of charges, i.e., current flows into the IGBT chip 10, a large current also flows through the wires 15a as a matter of course. This causes a voltage drop at the wires 15a due to the wiring resistance R15a. Thus, during the voltage drop, the voltage inputted to the gate terminal 14 from the drive circuit 24 is reduced by this voltage drop.
Accordingly, although the drive circuit 24 applies a voltage signal having a sufficient amplitude for driving the IGBT chip 10 between the gate terminal 14 and emitter terminal 15, a phenomenon occurs that a voltage having an amplitude smaller than that of the above-described voltage applied between the gate terminal 14 and emitter terminal 15 is applied between the gate electrode 11 and emitter electrode 12. As a result, light emission failure of the strobe light and a malfunction due to noise easily occur.
As a resolution to such drawbacks, a method has conventionally been employed in which the amplitude of a voltage signal generated by the drive circuit 24 is increased in advance taking the voltage drop due to the resistance R15a into consideration, thereby providing a margin for a driving voltage (threshold voltage) for the IGBT chip 10.
The configuration of the above-described conventional IGBT device for controlling a strobe light is already in common use, and the above description is based on the inventor's knowledge. He has not found any published document that specifically discloses the configuration.
On the other hand, there is a technique to be used for an inverter circuit for preventing an adverse influence from being exerted upon an overcurrent protection circuit due to wiring inductance in an IGBT module including the overcurrent protection circuit (e.g., Japanese Patent Laid-Open No. 8-162631 (hereinafter referred to as document 1); pp. 2-3, FIGS. 1 and 2). The overcurrent protection circuit is connected to an emitter electrode inside the IGBT module. Potential variations at the emitter electrode due to load current variations (di/dt) in an inverter circuit and wiring inductance of the IGBT module cause the overcurrent protection circuit to malfunction. The document 1 describes providing the IGBT module with an auxiliary emitter terminal that connects the emitter electrode to a ground terminal of a gate drive circuit, thereby preventing a malfunction of the overcurrent protection circuit due to potential variations at the emitter electrode.
In digital camera equipment, for example, there is a trend toward lower supply voltage for internal circuits of such equipment with lower power consumption in recent years. For instance, a value of supply voltage for internal circuits in conventional digital cameras has mainly been 5.0V, which, however, is being changed to 3.3V recently. A decrease in supply voltage for a circuit naturally imposes a limit on increasing an output voltage from the drive circuit 24. It is thus becoming difficult to implement the method of increasing a margin for a voltage signal.
Therefore, a drive voltage for an IGBT chip for controlling a strobe light, which has mainly been 4.0V is required to be reduced to about 2.5V. This allows a large margin for a drive voltage for an IGBT chip even with a low supply voltage. However, merely decreasing a drive voltage for the IGBT chip disadvantageously degrades the IGBT chip itself in noise immunity.